De Gea is getting old and maybe that's why Manchester United tried to decrease his salary for the next season since is a famous payer and achieved many goals for this team. It was not good to ask this from De Gea however I think it was not about salary but that's mostly because of Ten Hag, he prefers to work with the goalkeeper he had at Ajax
Initially his salary was going to be cut by 100k per week and De Gea agreed somehow Manchester United wanted to renew his contract again and here De Gea did not agree and decided to move, we don't know if he moved because his salary were lower or he felt disrespected as a legend. actually Manchester United were criticized for what they did to De Gea but their decision is fixed and does not change anything.